U.S. Pat. No. 3,928,196 discloses the use of copolymers of 2-acrylamido-2-methylpropylsulfonic acid and acrylic acid as scale inhibitors.
U.S. Pat. No. 4,640,793 discloses admixtures containing carboxylic acid/sulfonic acid polymers as scale and corrosion inhibitors.
U.S. Pat. No. 4,618,448 discloses the use of polymers comprising an unsaturated carboxylic acid, an unsaturated sulfonic acid and an unsaturated polyalkylene oxide as scale inhibitors.
Japanese No 57-084794 discloses the use of copolymers of acrylic acid and allyl polyethylene glycol as scale inhibitors.
European patent application 84301450.7 discloses carboxylic acid/sulfonic acid copolymers in combination with organic phosphonates as scale inhibitors.
The above listed references do not disclose or suggest the criticality of pH, alkalinity, calcite saturation and/or temperature with respect to the control of calcium carbonate scaling. Therefore, these references do not disclose or suggest the instant invention, which relates to the control of calcium carbonate scaling under severe pH, alkalinity, calcite saturation and/or temperature conditions.
Most industrial waters contain alkaline earth metal cations, such as calcium, barium, magnesium, etc. and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reactions products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction products, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change in pH, pressure or temperature, or the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of a water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes and harbors bacteria. This scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Since chelation is a stoichiometric reaction, it necessitates adding stoichiometric amounts of a chelating or sequestering agent relative to the cation concentration. These amounts are not always desirable or economical.
Almost 50 years ago, it was discovered that certain inorganic polyphosphates will prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating. By polyphosphates, we mean phosphates having a molar ratio of metal oxide: P.sub.2 O.sub.5 between 1:1 and 2:1.
When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for stoichiometric binding of the scale-forming cation, it is said to be present in "threshold" amounts. See for example, Hatch and Rice, "Industrial Engineering Chemistry", Vol. 31, pages 51 to 53 (Jan. 1939); Reitemeier and Buehrer, "Journal of Physical Chemistry", Vol. 44, No. 5, pages 535 to 536 (May, 1940); Fink and Richardson, U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305.
Generally, sequestration takes place at weight ratios of threshold active compound to scale-forming cation components of greater than about 10:1, depending on the anion components in the water. Threshold inhibition generally takes place at weight ratios of threshold active compound to scale-forming cation components of less than about 0.5:1.0.
Certain water soluble polymers, including groups derived from acrylamide and acrylic acid, have been used to condition water containing scale-forming cation components of less than about 0.5:1.0.
Under high pH and/or alkalinity conditions, the precipitation of calcium carbonate in aqueous systems is especially difficult to control. Generally, operators add acid to lower the pH and consume alkalinity to prevent calcium carbonate scaling. This is necessary since traditional scale inhibitors for calcium carbonate, such as hydroxyethylidene diphosphonic acid (HEDP), are ineffective in highly alkaline waters. For example, HEDP is ineffective because it reacts with calcium at high pH's to form a calcium/phosphonate scale.
The inventors have discovered a method for controlling calcium carbonate scaling in an aqueous system having high pH, alkalinity and/or calcite saturation values utilizing compositions which contain water soluble polymers prepared using carboxylic acid, sulfonic acid and polyalkaline oxide moieties and water soluble phosphonates. Optionally, water soluble carboxylic polymers may be added. Alone, none of the components provide high threshold inhibition (less than 90%), regardless of dosage. The instant compositions, however, provide greater than 90% threshold inhibition in severe waters.